The present invention relates generally to wireless communication systems and particularly relates to channel estimation in wireless communication systems that employ transmitter diversity.
Reliability represents a critical measurement in assessing the performance of any communication system, including the many types of wireless communication systems in use today. In wireless environments, achieving reliable communication requires overcoming the many challenges associated with non-ideal reception conditions, and particularly with overcoming or compensating for changing channel characteristics.
With the complex encoding and wide band channels employed by some types of wireless communication systems, successful signal reception and information recovery often depends on effective channel equalization wherein a receiver compensates a received signal for the characteristics of the channel through which it was received. Channel equalization reduces block or bit error rates in the received signal to acceptable levels and, indeed, permits successful communication under channel conditions that would otherwise prevent reliable communication.
Channel equalization requires that the receiver xe2x80x9clearnxe2x80x9d the characteristics for the reception channel, which is generally taken to be the signal path between the transmitter and the receiver. In blind estimation techniques, the receiver determines the channel characteristics based on decoding information from the received signal without benefit of a priori knowledge of the transmitted information. Training may be somewhat simplified and improved, where possible, by transmitting a known sequence during a portion of the received signal. Channel estimation is simplified in this scenario in as much as the receiver knows what sequence of values it should have received during the training portion of the received signal and can thus compare what it actually received with what it should have received.
Another problem of significant concern in wireless communication systems is channel fading. When there is relative motion between the transmitter and receiver, the radio signal propagation path changes as the nature and position of obstructions between them changes. This causes abrupt changes in received signal strength, and can result in a complete loss of the received signal under extreme fading conditions. Transmit signal diversity, wherein two or more antennas transmit the same information signal, represents one approach to combating fading problems. With multiple transmit antennas there is a greater likelihood that at least one of the transmitted signals from the multiple antennas will not be completely faded, thus allowing successful signal reception at the receiver.
However, transmit diversity complicates received signal equalization because the receiver must develop accurate channel estimates for each transmit path or channel. One approach to this task involves transmitting separate training sequences from each transmit antenna. To facilitate estimation of these multiple channel responses at the receiver, the different training sequences transmitted from the multiple antennas must have good auto- and cross-correlation properties. Finding an adequate number of training sequences with the requisite correlation qualities is difficult at best, owing to the limited length of the training sequences that can be used and the limited xe2x80x9calphabetxe2x80x9d from which distinct training values may be formed. The difficulty in finding suitable training sequences increases with the number of transmit antennas used.
A wireless communication system employing transmit diversity uses multiple time intervals or slots in a wireless communication system to perform channel estimation between multiple transmit antennas and a given receiver using a single training sequence. Rather than using a potentially difficult-to-find set of distinct training sequences, one for each transmit antenna, each transmit antenna broadcasts the same training sequence, but scaled and signed in each time interval according to a corresponding value in a scaling value matrix. The receiver performs basic channel estimation in each of the time intervals as if there were only one transmit antenna. A channel estimate for each transmit antenna may then be determined at the receiver by properly combining the channel estimates obtained across all time intervals.
The scaling value matrix is defined where each column corresponds to a particular one of the multiple transmit antennas, and where the rows correspond to successive time intervals. The column values are defined such that each column is orthogonal with respect to all other columns. Thus, the jth antenna at the kth time interval or slot transmits the training sequence scaled by the matrix element bkj. At the receiver, a combined channel estimate is developed for each received signal time slot as if there were only one transmit antenna. The receiver then sums the combined estimates for each antenna based on the same scaling value matrix used to scale the transmit signal. This estimate may itself comprise a set of estimates corresponding to multipath signal components from each of the antennas. The orthogonal properties of the scaling value matrix allows the receiver to cancel signal interference from the other transmit antennas when combining the composite channel estimates for individual ones of the transmit antennas.